(CNSNews.com) – A month-long standoff between China and the Philippines over a disputed area in the South China Sea has worsened, with Chinese state media raising threats of military action while Beijing warns that its citizens in the Philippines may be at risk because of nationalistic sentiment there.

State Department spokeswoman Victoria Nuland said the U.S. was “urging restraint from all parties” and “discouraging any kind of escalation of tensions.”

Philippine Defense Secretary Voltaire Gazmin this week drew attention to comments by Secretary of State Hillary Clinton and Defense Secretary Leon Panetta – during ministerial talks in Washington on April 30 – to the effect that the U.S. was not taking sides in the territorial dispute but would honor its 1951 Mutual Defense Treaty with the Philippines.

Nuland on Thursday characterized the treaty assurance as routine.

“In the context of the visit here, as we always do when we meet with Philippine leaders, we reconfirmed our commitment to the Mutual Defense Treaty,” she said.

Article four of the treaty states in part, “Each Party recognizes that an armed attack in the Pacific Area on either of the Parties would be dangerous to its own peace and safety and declares that it would act to meet the common dangers in accordance with its constitutional processes.”

Article five adds that “an armed attack on either of the Parties is deemed to include an armed attack on the metropolitan territory of either of the Parties, or on the island territories under its jurisdiction in the Pacific or on its armed forces, public vessels or aircraft in the Pacific.”

Chinese and Philippine non-military ships have been facing off since early April in a resource-rich area known as the Scarborough Shoal, about 130 miles away from the Philippines mainland, and at least five times that far from the nearest significant Chinese territory, Hainan island.

China says the shoal, which it calls Huangyan, has been Chinese for centuries; Philippine authorities say the area, which it calls Panatag, has appeared as part of the Philippines on maps dating back to the 1700s.

On April 9 Philippine authorities seized what they said was illegally harvested marine life from Chinese ships, but when a Philippine Navy warship tried to tow the Chinese vessels they were blocked by two Chinese ships.

At least a dozen Chinese ships are now in the disputed area, along with some Philippine vessels, according to Philippine officials.

Manila says the shoal falls well inside its 200 nautical mile exclusive economic zones (EEZ), as recognized under the U.N. Convention on the Law of the Sea. Beijing claims all of the South China Sea, and has additional disputes there with Vietnam, Malaysia, Taiwan and Brunei, largely over the presence of natural resources in areas where countries’ EEZs overlap.

China argues that the shoal’s proximity to the Philippines is irrelevant.

“There is no such principle in international law that determines territorial ownership by geographic distance,” the Xinhua news agency quoted senior State Oceanic Administration official Zhang Haiwen as saying.

Zhang pointed out that the British Channel Islands lie just off the French coast but do not belong to France; while France has territories in the Atlantic and even the Pacific.

‘No room for bargaining’

China's Vice Foreign Minister Fu Ying met with a Philippine diplomat in Beijing early this week but was quoted by Xinhua afterwards as saying China was not optimistic about the situation and “has also made all preparations to respond to any escalation of the situation by the Philippine side.”

Media outlets meanwhile ratcheted up rhetoric on the issue, painting China as the aggrieved party that may be forced – reluctantly – to resort to force to protect its interests.

“No matter how willing we are to discuss the issue, the current Philippine leadership is intent on pressing us into a corner where there is no other option left but the use of arms,” the state-run China Daily said in an editorial, while the Communist Party-affiliated Global Times warned that the international community should not to be “completely surprised” if a military confrontation ensues.

The People’s Liberation Army mouthpiece, PLA Daily, published an article Thursday entitled “Don’t Attempt to Take Away Half an Inch of China’s Territory.”

“We want to say that anyone’s attempt to take away China’s sovereignty over Huangyan Island will not be allowed by the Chinese government, people and armed forces,” it said. “If one mistakes China’s kindness for weakness and regards China as a ‘paper dragon’ as instigated by some onlookers, he is terribly wrong.”

“China always adheres to friendly relationships with its neighbors,” a Xinhua commentary said. “But territorial sovereignty is a core interest for China and there is no room for bargaining.”

Accusing Manila of failing to “curb its improper coveting of China’s marine territory,” it said the Philippines’ actions “can bring nothing but harm to the country’s economy.”

That implicit warning of economic pressure took further shape with the announcement Thursday that Chinese tourism companies have suspended tours to the Philippines.

China’s embassy in Manila issued a notice advising citizens in the country to avoid going outdoors, ahead of planned protests by activist groups on Friday.

“China hopes the Philippines will not take any actions to magnify the dispute in a way that may affect the relationship between the two countries,” he said. “China remains unchanged in insisting on diplomatic dialogue to solve the Huangyan island dispute. We urge the Philippine side to make a positive response, and move back on the right track.”